Mass Transport Losses Research Articles

In-Depth Analysis of the Impact of Ionomer Content on PEMFC Degradation By Combining Electrochemical Characterization Techniques and Transmission Line Modeling

The future success of the polymer electrolyte membrane fuel cell (PEMFC) technology depends on a further increase in performance and durability as well as a decrease in costs. The catalyst layer plays an important role in these aims, as the employed catalyst, carbon and ionomer materials and their composition strongly influence these three success factors.One key parameter, which has a strong effect on the cathode catalyst layer (CCL), is the ionomer to carbon (I/C) ratio. The I/C ratio has shown a direct impact on mass transport processes, performance [1] and cold start behavior [2] of PEMFCs. It also has an influence on the degradation during cycling of relative humidity [3], temperature [4] and high potential cycling [5]. One of the most crucial operating states of PEMFCs is the start-up/shut-down condition, during which high potentials arise due to an oxygen-hydrogen front, which lead to a corrosion of the electrode carbon structure. Accelerated stress tests (ASTs) commonly emulated this condition by cycling at high potentials (1.0-1.5 V) [6,7]. As the I/C ratio strongly affects the microstructure and mass transport processes in PEMFCs, catalyst layer reconstruction due to carbon corrosion and its effect on mass transport losses may have an interaction with the I/C ratio.This investigation aims to analyze the influence of the I/C ratio on the carbon corrosion behavior by applying high potential cycles using an incremental 1 cm2 single-cell setup. The incremental cell setup enables a gradient-free operation of the cell concerning humidification, gas composition, temperature, pressure and degradation. For the in-depth analysis of the carbon corrosion behavior, several electrochemical in- and ex-operando techniques are combined. The electrochemical impedance spectroscopy (EIS) under H2/air conditions combined with the distribution of relaxation times (DRT) method (see figure 1) and the subsequent fit of a physicochemically motivated transmission line model (TLM) enable a deconvolution and quantification of the ohmic, charge transfer, mass transport and ionic resistances in the CCL [8]. To better understand the degradation mechanisms within the CCL, these results are complemented by EIS under H2/N2 conditions, cyclic voltammetry (CV) and limiting current measurements separating the molecular diffusion (GDL/gas channel) from the Knudsen/film diffusion (MPL/CCL/ionomer and water film) resistances [9]. The combined results from different in- and ex-operando characterization techniques enable us to obtain a more comprehensive understanding of the dominating degradation mechanisms and the interaction with varying I/C ratios. The insights gained from this study can guide future material design for enabling fuel cell commercialization.

Effects of Selected Fuel Cell Operating Conditions on the Distribution of Liquid Water in the Cathode Gas Diffusion Layer

Effective water management is required to enable polymer electrolyte fuel cells (PEFCs) to operate efficiently, particularly at the high current densities needed to make them relevant for transportation and stationary applications. The presence of liquid water can influence the performance of a PEFC when the rate of water production exceeds the rate of water removal [1]. More specifically, accumulation of water within the cell becomes problematic when the pores within the gas diffusion layer (GDL) become filled, resulting in restriction of the transport of reactants to the catalyst layer [2]. Identifying operational conditions that minimize mass transport losses associated with excess water accumulation is crucial for improving the performance of PEFCs, especially at high current densities. Additionally, GDL design plays an important role in reducing mass transport losses, with thermal conductivity being a key factor influencing the distribution of liquid water.In this work, miniaturized fuel cells were operated under various conditions and imaged in-operando using X-ray Computed Tomography (XCT) [3]. This imaging technique facilitates the evaluation of the distribution of liquid water at steady state over a range of operational conditions. The influence of current density on liquid water distribution was assessed at selected cell temperatures between 40°C and 70°C. The results of this study showed high saturations under both the land and the channel regions at 50 °C. However, the channels appeared to be dry at 70 °C, with liquid water present only under the lands. These data were compared to predictions generated using a one-dimensional (1-D) model of the through-plane GDL, which considers specific operating conditions and GDL characteristics, such as thermal conductivity [4]. These results demonstrate that the 1-D model can be used to predict the presence or absence of liquid water within the GDL (and, by extension, PEFC performance) under a broad range of operating conditions. Hence, the 1-D model may be useful for focusing future laboratory investigations. Keywords – operando, X-ray tomographic microscopy, GDL, water visualization Acknowledgement Funding for this research was provided by the Natural Sciences and Engineering Research Council of Canada, Ballard Power Systems, Simon Fraser University, Canada Foundation for Innovation, British Columbia Knowledge Development Fund, and Canada Research Chairs.

Tafel Slope Estimation of Electronic Resistance Laden Catalyst Layer in PEMFC Using Polarization Curve Correction Methodology

The higher cost of PEMFC systems proves to be a major bottleneck in their adoption. Platinum being a costly material (used as a catalyst in PEMFC) contributes significantly (42%) to the overall cost of the PEMFC stack.1 The majority of the Platinum catalyst is loaded on the cathode side due to the slow nature of the oxygen reduction reaction (10-8 A/cm2 for ORR). Hence, alternative options for the cathode catalysts are being persued as a potential alternative for platinum metal group-free (PGM-free) catalyst to improve the economics of such systems.2 The sluggish kinetics of PGM-free catalysts requires much thicker CCL (compared to the Pt catalyst) to have comparable fuel cell performance at low currents.3 Thicker CCLs will lead to poorer protonic and electronic conduction.Voltage breakdown analysis from the polarization curve (Cell voltage vs Current density) is quite common in literature and is used to gather selective insights into the working of PEMFC. Work by Neyerlin et al. mathematically quantified the effect of proton conduction on the polarization curve.4 In their work, they also proposed a systematic correction methodology to retrieve the Tafel Slope by correcting the polarization curve for proton conduction and other ohmic resistances (membrane and external resistances). It is to be noted that the correction approach presented by Neyerlin et al. works well for systems with neglible electronic resistance (since it was designed for Platinum based CCLs which are thin and highly electronically conducting). However, for PGM-free catalysts which are thicker and poorer electronic conductors than Pt-based CCLs, contribution from both electronic and protonic conduction will affect the polarization curve.We propose a method to correct the polarization curve in PEMFC for effective total transport loss (both electronic and protonic) to get back the Tafel slope of the oxygen reduction reaction on the cathode side. Further, we demonstrate the approach for two different CCLs made with commercially available catalysts named PMF and in-house developed catalysts named HM (presented in Li et al)5. Note that this work assumes that the mass transport losses are not present (which is reasonable assumption in H2/O2 system) and assumes that gas cross-over effects (due to the crossing of gases through the membrane to the other side of the PEMFC) are not current dependent during fuel cell operations.Figure 1 shows the correction steps for hypothetical data generated from bvp-5C simulations (for an electronic resistance laden catalyst layer) which is treated as an actual polarization curve (orange curve represented as Ecell). In this work, we develop a semi-analytical expression for effective transport loss (represented by cyan shaded in Figure 1) which can be used to correct the (corrected for membrane proton conduction and other external electronic resistance referred to as Rmem+ext) to obtain a curve which, when fitted with the Tafel kinetics equation gives the value the Tafel slope of that CCL. References S. T. Thompson et al., Solid State Ion., 319, 68–76 (2018).Lefèvre Michel, Proietti Eric, Jaouen Frédéric, and Dodelet Jean-Pol, Science (1979), 324, 67–71 (2009).F. Jaouen et al., Johnson Matthey Technology Review, 62, 231–255 (2018).K. C. Neyerlin, W. Gu, J. Jorne, A. Clark, and H. A. Gasteiger, J. Electrochem. Soc., 154, B279 (2007).Y.-S. Li, D. Menga, H. A. Gasteiger, and B. Suthar, J. Electrochem. Soc., 170, 094503 (2023). Figure 1

Structure-Property Relationships for Thin-Ilm Nafion: Model-Predictive Design of Low-Pt PEMFC Catalyst Layers

Proton exchange membrane fuel cells (PEMFCs) are a promising means to decarbonize the global vehicle fleet, but commercialization is limited by costly Pt catalysts, among other challenges. PEMFC designs with low Pt loading typically suffer anomalous performance losses. Previous studies have agreed that poor performance with low Pt-loading is attributed to slow transport through the nano-thin Nafion ionomer found in the catalyst layer (CL). Although it is known that the properties of the Nafion thin films in the CL vary from those previously measured for thicker membranes, research has not yet definitively connected CL polymer properties with PEMFC limitations in low Pt-loaded cells, nor found a means to address the limitations for low cost, higher-performance PEMFCs.This presentation covers parallel efforts to identify and address limiting processes in low-Pt-loaded PEMFC cathode CLs. We begin with an extensive investigation of thin-film Nafion structure-property relationships, as an analog for CL Nafion thin-films. We model nm-scale transport in thin-film Nafion to develop structure-property relationships for thin-film Nafion, based on in situ neutron reflectometry [2], and conductivity measurements [3] on thin film Nafion. These include equilibrium measurements for varying thicknesses and substrates, as well as novel in operando measurements of thin-film Nafion with an H2O flux across the film. Results suggest that conductivity in CL Nafion is a function of not just temperature and relative humidity (as in bulk Nafion membranes), but also film thickness and proximity to the substrate. These insights result in a model that calculates the conductivity as a function of temperature, local water uptake, film thickness, and underlying substrate (Pt or carbon).Simultaneously, we have conducted continuum-scale numerical simulations of PEMFC catalyst layers, to determine the necessary level of detail for predictive yet efficient numerical simulation for device design. Incorporating these structure-property models into a continuum-scale PEMFC cathode half-cell model to locally resolve proton conductivity and O2 diffusion improves model fits to PEMFC data with varying Pt loading [4]. The fits provide new insights into the coupling between kinetic, mass transport, and Ohmic losses in low-Pt-loaded PEMFC cathodes and show that ohmic losses related to proton transport in thin film Nafion play a key role in low-Pt loaded cells. Based on these new fits to low-Pt loaded cells, we present a parametric study of PEMFC CL design parameters with low Pt loading. Incorporating multi-scale modeling of Nafion transport processes enhances the predictive model capabilities and unlocks new insights into CL design. In particular, results suggest that functionally graded catalyst layer designs could significantly improve PEMFC power density with low Pt loading [4]. Weber, A. and Kusoglu, A. Mater. Chem. A. 2014. DOI: 10.1039/c4ta02952fDeCaluwe, S.C. et al. Nano Energy 2018.DOI: 10.1016/j.nanoen.2018.01.008Paul, D.K. et al. Electrochem. Soc. 2014. DOI: 10.1149/2.0571414jesRandall, C.R. and DeCaluwe, S.C., Electrochem. Soc. 2022. DOI: 10.1149/1945-7111/ac8cb4

Exploring Reference Electrodes in Polymer Electrolyte Membrane Water Electrolysis: Analysis of Design Challenges and Review of Characterization Results for Oxygen and Hydrogen Evolution Reaction Kinetics

Clean hydrogen from renewable energy sources via proton exchange membrane water electrolyzers (PEMWE) has great potential to decarbonize various sectors and integrate intermittent renewables into the energy mix (1). However, its large-scale deployment faces significant hurdles, mainly due to the high cost of materials like platinum and iridium used as electrocatalysts (2). Improving the characterization of catalyst layers in PEMWE helps to address this, as a better understanding of the efficiency and durability of the catalyst layers drives research advancements and facilitates cost-reduction efforts.Current characterization tools have limitations in providing realistic insights into catalyst performance in PEMWE. This contribution explores using reference electrodes (REs) to address this gap. REs offer the advantage of measuring the proton potential within the system, which helps to optimize performance and understand degradation mechanisms. However, integrating REs into PEM cells presents challenges, such as the impact of the RE device on the cell or its accuracy (3–5).This study presents the integration of two reference electrode (RE) setups, a salt bridge RE (SBRE) and a dynamic hydrogen RE (DHE), into a standardized PEMWE test cell. The focus is on overcoming design optimization challenges and mitigating performance impacts. Among these challenges is the influence of ionomer treatment of the porous transport layer (PTL), an integral part of the SBRE design, which affects cell performance. The impregnation of the PTL can result in increased ohmic, mass transport and other losses (5). The Figure below shows the full cell polarization curve for both pristine PTLs and PTLs subject to various degrees of impregnation. A clear pattern can be observed where lower impregnation volumes result in cell performance closer to that of the unimpregnated counterpart. An optimal impregnation volume of 1 µL of a 5 wt% electrolyte solution is identified. This adjustment minimizes the impact on cell performance while ensuring the necessary ionic connectivity is maintained.Additionally, another challenge lies in electrochemical impedance measurements (EIS) for half-cells. This is addressed by integrating improved EIS settings that strategically avoid high frequencies prone to inductance, enabling accurate HFR measurements in half-cells for subsequent kinetic analyses. In addition to these challenges, further obstacles, such as potential gradient across the salt bridge and RE precision, are also addressed. These challenges are thoroughly analyzed both theoretically and experimentally, and corresponding solutions are presented.Building on the thorough analysis of the RE setups and the implemented solutions and enhancements, experimental investigation and kinetic analysis are carried out. An overview of the findings is given, which includes OER and HER kinetic parameters, investigates temperature effects on kinetics, and elucidates aspects of degradation. In addition, the application of the Arrhenius equation to analyze the temperature dependence of the exchange current density provides insight into the activation energies for half-cell reactions.References U.S. Department of Energy, Hydrogen Production Processes, https://www.energy.gov/eere/fuelcells/hydrogen-production-processes.S. Krishnan, V. Koning, M. Theodorus de Groot, A. de Groot, P. G. Mendoza, M. Junginger and G. J. Kramer, International Journal of Hydrogen Energy, 48(83), 32313–32330 (2023).L. V. Bühre, A. J. McLeod, P. Trinke, B. Bensmann, M. Benecke, O. E. Herrera, W. Merida and R. Hanke-Rauschenbach, J. Electrochem. Soc. (2023).A. J. McLeod, L. V. Bühre, B. Bensmann, O. E. Herrera and W. Mérida, Journal of Power Sources, 589, 233750 (2024).L. V. Bühre, S. Bullerdiek, P. Trinke, B. Bensmann, A.-L. Deutsch, P. Behrens and R. Hanke-Rauschenbach, J. Electrochem. Soc. (2022). Figure 1

Utilizing Distribution of Relaxation Times Analysis for Water Content Management in Anion Exchange Membrane Fuel Cells

Amid escalating concerns over air pollution and energy scarcity, Anion Exchange Membrane Fuel Cells (AEMFCs) are increasingly recognized as a viable solution for power generation, offering high efficiency, and zero emissions. Water plays a critical role within these systems as a reactant, consumed at the cathode and generated at the anode. This process can lead to disparities in water content, potentially compromising the cell's performance. Therefore, it is imperative to maintain optimal water distribution to facilitate effective charge transport and enhance electrochemical reaction kinetics. Consequently, devising a robust water management strategy is essential to ensuring the sustained operation and longevity of AEMFCs.Characterizing the water conditions at the anode and cathode under various operational states (normal, drying, or flooding) is a pivotal step underpinning effective water management strategies. Electrochemical Impedance Spectroscopy (EIS) provides a valuable tool for examining various resistances associated with kinetics, mass transport, and ohmic losses across different operational timescales, thereby facilitating a detailed assessment of water transport dynamics. Traditional analyses of EIS data often depend on equivalent circuit models, which require predefined assumptions and the selection of initial components. However, this study adopted the Distribution of Relaxation Times (DRT) methodology for its exceptional capability to disaggregate components, enabling a more nuanced analysis of the polarization processes. The DRT methodology was applied to scrutinize the patterns of loss and variation within each polarization process under varying water flooding and drying conditions at the anode and cathode. Moreover, to deepen our comprehension of how water content influences mass transport and electrochemical reaction kinetics, a three-dimensional multi-physics model for AEMFCs was developed. This model comprehensively integrates considerations of water phase transitions, heat and mass transfer, as well as the transport of liquid and dissolved water, alongside electrochemical reactions. These efforts collectively forge a comprehensive and systematic framework to advance the application of the distribution of relaxation times in optimizing fuel cell performance.

Micro-Scale Thermal Partial Oxidation Reforming of Liquid Fuels and Solid Oxide Fuel Cell Performance and Stability Analysis

Thermal partial oxidation of hydrocarbons to hydrogen and carbon monoxide (i.e., synthesis gas) is limited for conventional scale reactors due to soot formation under higher fuel/air equivalence ratios. Recent research into micro-scale (<4 mm diameter) thermal partial oxidation reactors with a controlled temperature of 800-900 °C has shown promise for conversion to synthesis gas without soot formation. Direct integration of Solid Oxide Fuel Cells (SOFCs) with these micro-scale thermal partial oxidation reactors has been proposed for portable power generation systems. In this work, the micro-scale thermal partial oxidation of methanol/air is investigated at fuel/air equivalence ratios from 1-5 and at a temperature of 800 °C. High conversion rates to synthesis gas are measured with a gas chromatograph for equivalence ratios between 1.5 and 3, with the concentration of hydrogen exceeding 10% and approaching chemical equilibrium predictions. A micro-tubular SOFC is integrated with the micro-scale thermal partial oxidation reformer to electrochemically oxidize the synthesis gas while generating power. SOFC polarization losses are investigated at different equivalence ratios and temperatures of the reformer with results indicating the mass transport losses decrease significantly as the equivalence ratio increases. Peak power densities occur when the synthesis gas concentration is maximized. Short term testing of the SOFC is stable no carbon deposition on the Ni-YSZ anode.

Compression of Porous Transport Layers Enhanced Bubble Transport in Polymer Electrolyte Membrane Water Electrolyzers

At high current densities, oxygen bubble formation limits the performance of polymer electrolyte membrane (PEM) water electrolyzers, particularly due to bubble accumulation at the porous transport layer (PTL)/catalyst layer (CL) interface. Efficient bubble removal from the PTL/CL interface is therefore crucial for PEM water electrolyzers1,2. Bubble transport at the PTL/CL interface is dominated by mechanical compression 3,4. However, it is still unclear how mechanical compression impacts bubble transport at the PTL/CL interface, due to difficulties in capturing the multiphase flow and differentiating liquid/gas in the pores of the titanium PTL. Thanks to its spatial resolution and high transmissivity for titanium, neutron imaging enables visualization of the bubble distribution in the PTL hence allowing for the identification of the role of compression in the bubble transport.We conducted operando neutron imaging for PEM water electrolyzers and examined the relationship between compression ratio and electrochemical performance. Neutron imaging results showed that the through-plane bubble distributions became more homogeneous as compression increased. Additionally, pore network modelling simulations indicated that compression-induced catalyst coated membrane intrusion homogenized the bubble distribution in the PTL. We identified an optimal compression condition for enhanced interfacial bubble transport that can contribute to the next-generation material development to improve the efficiency of PEM water electrolyzers at high current densities with low mass transport losses. References J. K. Lee and A. Bazylak, Joule, 5, 19–21 (2021).S. Yuan et al., Prog. Energy Combust. Sci., 96, 101075 (2023).T. Schuler, T. J. Schmidt, and F. N. Büchi, J. Electrochem. Soc., 166, F555–F565 (2019).A. Martin et al., J. Electrochem. Soc., 169, 014502 (2022).

Strategies for gas management in the PEM water electrolyzer anode

Proton Exchange Membrane Water Electrolyzers (PEMWE) are viewed as a promising pathway to generate green hydrogen. The need to reduce the cost of the produced hydrogen motivates efforts to increase the operating current density, which requires better management of the oxygen bubbles released within the PEMWE's flooded anode. Prior research has shown that the buildup of gas bubbles within the PEMWE anode increases mass transport resistance and reduces performance. This study employs a 10 cm2 single-cell PEMWE operating at over 2.5 A/cm2 with a transparent single-serpentine anode to visualize bubble production and transport under varying operating conditions such as the cell's orientation with respect to gravity, anode water supply rate, and the effect of adding surfactant to the anode water supply. These strategies are studied to optimize the management of oxygen bubbles in the anode channels of a PEMWE operating at 80 °C. It was found that the Anode Top channel orientation with respect to gravity was optimal in terms of assisting with bubble evacuation and improved performance. Increasing the anode water supply rate also facilitated bubble detachment and alleviated mass transport losses. Lastly, the addition of surfactant decreased the water contact angle from 83° to 57° which was found to aid bubble detachment from the porous transport layer and improve current stability.

Honeycomb‐Structured IrOx Foam Platelets as the Building Block of Anode Catalyst Layer in PEM Water Electrolyzer

AbstractAchieving robust long‐term durability with high catalytic activity at low iridium loading remains one of great challenges for proton exchange membrane water electrolyzer (PEMWE). Herein, we report the low‐temperature synthesis of iridium oxide foam platelets comprising edge‐sharing IrO6 octahedral honeycomb framework, and demonstrate the structural advantages of this material for multilevel tuning of anodic catalyst layer across atomic‐to‐microscopic scales for PEMWE. The integration of IrO6 octahedral honeycomb framework, foam‐like texture and platelet morphology into a single material system assures the generation and exposure of highly active and stable iridium catalytic sites for the oxygen evolution reaction (OER), while facilitating the reduction of both mass transport loss and electronic resistance of catalyst layer. As a proof of concept, the membrane electrode assembly in single‐cell PEMWE based on honeycomb‐structured IrOx foam platelets, with a low iridium loading (~0.3 mgIr/cm2), is demonstrated to exhibit high catalytic activity at ampere‐level current densities and to remain stable for more than 2000 hours.

Honeycomb-Structured IrOx Foam Platelets as the Building Block of Anode Catalyst Layer in PEM Water Electrolyzer.

Achieving robust long-term durability with high catalytic activity at low iridium loading remains one of great challenges for proton exchange membrane water electrolyzer (PEMWE). Herein, we report the low-temperature synthesis of iridium oxide foam platelets comprising edge-sharing IrO6 octahedral honeycomb framework, and demonstrate the structural advantages of this material for multilevel tuning of anodic catalyst layer across atomic-to-microscopic scales for PEMWE. The integration of IrO6 octahedral honeycomb framework, foam-like texture and platelet morphology into a single material system assures the generation and exposure of highly active and stable iridium catalytic sites for the oxygen evolution reaction (OER), while facilitating the reduction of both mass transport loss and electronic resistance of catalyst layer. As a proof of concept, the membrane electrode assembly in single-cell PEMWE based on honeycomb-structured IrOx foam platelets, with a low iridium loading (~0.3 mgIr/cm2), is demonstrated to exhibit high catalytic activity at ampere-level current densities and to remain stable for more than 2000 hours.

Liquid and gas permeabilities of nanostructured layers: Three-dimensional lattice Boltzmann simulation

Liquid water and gas permeabilities in nanostructured catalyst layers (CLs) (pore size ∼10–150 nm) and microporous layers (MPLs) (pore size ∼50 nm-5 μm) are crucial to reduce the mass transport loss in proton exchange membrane fuel cells (PEMFCs). Experimental measurement of their permeabilities poses great challenges due to their brittle and thin characteristics. Presently, considerable discrepancy exists in the reported permeability values for CLs and MPLs in the literature with variation spanning several orders of magnitude. This study digitally reconstructed various nanostructures of similar pore size as CLs and MPLs to calculate their permeabilities using the Lattice Boltzmann Method (LBM) based on no-slip (e.g. for liquid water flow in MPL and CL's secondary pores) or slip condition (e.g. for gas flow in MPL). Analysis was performed to evaluate the range of pore size in nanostructures for the two boundary conditions. A comprehensive investigation was conducted under various parameters such as the Reynolds number (Re), sphere diameter, porosity, and pore structure. The results revealed that permeability exhibits an increasing trend with the diameter and porosity. The predicted permeability of randomly arranged spheres agrees well with established correlations. The random sphere structure demonstrates higher permeability than their body-centered cubic (BCC) and face-centered cubic (FCC) counterparts. Under slip boundary condition, the permeability was enhanced, compared to the no-slip condition. This study's novelty lies in using distinct boundary conditions to assess the permeabilities of liquid and air, based on flow pattern analysis in the CL and MPL pore structures. A new empirical correlation describing the influence of the Knudsen number (Kn) on the slip permeability was developed, exhibiting consistency with simulation across the entire porosity spectrum.

Influence of the Clamping Force and the Formation of Liquid Water Along the Channel on the Local Diffusion Processes in PEMFCs

A novel measurement setup, technique, and segmented model are introduced to spatially resolve mass transport losses along the channel in a PEMFC. The newly designed segmented cell enables not only the measurement of local current and voltage data but also the capture of local temperature and compression levels. By extending the commonly used limiting current method, the local mass transport resistance is determined as a function of local operating parameters. The local current and temperature are directly measured, while the operating conditions along the channel such as pressure, oxygen fraction, and relative humidity are obtained empirically from a 1-D differential model. Leveraging both measured and empirically-based simulation results, this comprehensive approach facilitates the characterization of mass transport resistance distribution along the gas channel. The learnings from this study provide valuable insight into the optimization of channel and materials design for enhancing large active area fuel cell performance and durability.

Influence of the Clamping Force and the Formation of Liquid Water Along the Channel on the Local Diffusion Processes in PEMFCs

A novel measurement setup, technique, and segmented model are introduced to spatially resolve mass transport losses along the channel in a PEMFC. The newly designed segmented cell enables not only the measurement of local current and voltage data but also the capture of local temperature and compression levels. By extending the commonly used limiting current method, the local mass transport resistance is determined as a function of local operating parameters. The local current and temperature are directly measured, while the operating conditions along the channel such as pressure, oxygen fraction, and relative humidity are obtained empirically from a 1-D differential model. Leveraging both measured and empirically-based simulation results, this comprehensive approach facilitates the characterization of mass transport resistance distribution along the gas channel. The learnings from this study provide valuable insight into the optimization of channel and materials design for enhancing large active area fuel cell performance and durability.

Investigation of Dynamic Water Cluster and Droplet Interactions in Polymer Electrolyte Fuel Cells using Operando X-ray Tomographic Microscopy

Efficient removal of the electrochemically produced water from the gas diffusion layer (GDL) in polymer electrolyte fuel cells is crucial for reducing mass transport losses and improving the efficiency at high current densities. Understanding the relationship between the water percolation through the GDL and droplet formation in the gas channel will allow the design of advanced GDL materials, which provide optimal water management. In this study, a catalyst-coated membrane with 8 individual active areas (0.06 mm2 each) is investigated using operando X-ray tomographic microscopy to study the transient development and interaction of multiple percolating water clusters in a GDL and droplet formation in the channel. The 4D imaging results at a time resolution of 1 Hz showed transient instabilities in the developed percolating water networks at various frequencies associated with break-through and spontaneous water drainage.

Asymmetric gas diffusion layers for improved water management in PGM-free electrodes

Proton-exchange-membrane fuel cells (PEMFCs) offer a long-term, carbon-emission free solution to the energy needs of the transportation sector. However, high cost continues to limit PEMFC commercialization. Replacing expensive platinum group metal (PGM) catalysts with PGM-free catalysts could reduce cost, but the low active site density of PGM-free catalysts necessitates the use of thick electrodes that suffer from substantial mass transport losses. In these thick PGM-free electrodes, effective water management and oxygen transport are crucial to achieve high performance. In this work, we investigate the role of anode and cathode gas diffusion layer (GDL) configurations in controlling water management. Asymmetric GDL configurations, in which the anode GDL exhibits higher permeability than the cathode GDL, showed higher performance compared to conventional symmetric configurations. Computational modeling showed that the improved performance is mainly due to improved water management, resulting in lower liquid water saturation and faster oxygen transport in the cathode.

Distinguishing Electrochemical Processes in PEM Electrolysis By Impedance Modeling – Distribution of Relaxation Times

Proton exchange membrane (PEM) water electrolysis is a key technology in hydrogen production to provide capacity and flexibility for renewable energy power sources. Various research topics for electrolyzer development focus on cost reduction, performance optimization and lifetime extension with minimum platinum group metal (PGM) loading required [1]. Understanding degradation mechanisms in PEM electrolysis cells becomes necessary for developing mitigation strategies in dynamic durability tests, which facilitates the importance of advanced electrochemical characterization methods.Electrochemical impedance spectroscopy (EIS) is a commonly used diagnostic technique in electrochemical systems. For impedance interpretation, equivalent circuit model (ECM) can be an option with prior knowledge about the system and appropriate circuit elements to prevent overfitting. Alternatively, distribution of relaxation times (DRT) provides a model-free pathway to address electrochemical mechanisms with different time constants and it has been applied to fuel cells, batteries and capacitors in the past years [2, 3]. Impedance deconvolution by DRT, however, has not been well adapted for PEMECs. In this study, electrochemical processes are successfully identified from the DRT profile by varying membrane electrode assembly (MEA) compositions and different operational conditions. In a single cell testing, four main peaks in DRT results associated with (1) mass transport losses (2) reaction kinetic (3) ionic conductivity within the catalyst layer and (4) ohmic properties at the interface of membrane and catalyst layers can be separated and quantified. Examination of electron microscopy provides further explanation of different MEA characteristics in performance analysis. This study verifies DRT is a promising tool to better understand PEM electrolysis processes and will be applied to degradation mitigation in the future.

Controlled Catalyst Layer Architectures for Fuel Cell Applications

The catalyst layer composition and architecture play a pivotal role in bridging the gaps between ex situ and in situ catalyst activities in fuel cells.1 The catalyst-support architecture strongly influences the catalyst distribution, ionomer distribution and mass transport losses, occurring at high current density.2 Herein, a novel architecture based on self-standing fiber-network has been fabricated and characterized.3 The resulting porous structure of such catalyst layer is expected to control mass transport limitation across both the interface of catalyst layer by increasing the number of Triple Phase Boundaries. Further platinum catalyst is deposited onto this fibrous support as well as the ionomer. The full electrode is characterized for its morphology and electrochemically verified in both ex situ as well as in situ configurations. References (1) Jiantao Fan, Ming Chen, Zhiliang Zhao, Zhen Zhang, Siyu Ye, Shaoyi Xu, Haijiang Wang & Hui Li, “Bridging the Gap between Highly Active Oxygen Reduction Reaction Catalysts and Effective Catalyst Layers for Proton Exchange Membrane Fuel Cells”, Nature Energy, 6, 475–486, (2021).(2) Nagappan Ramaswamy, Wenbin Gu, Joseph M. Ziegelbauer and Swami Kumaraguru, “Carbon Support Microstructure Impact on High Current Density Transport Resistances in PEMFC Cathode”, Journal of The Electrochemical Society ,167, 064515, 2020.(3) Giorgio Ercolano, Filippo Farina, Sara Cavaliere, Deborah J. Jones and Jacques Rozière, “Towards Ultrathin Pt Films on Nanofibers by Surface-Limited Electrodeposition for Electrocatalytic Applications”, Journal of Materials Chemistry A , 5, 3974–3980, 2017.

Identification of Local Overpotentials and Their Spatial Distributions in Polymer Electrolyte Water Electrolyzers

Polymer electrolyte water electrolyzers (PEWEs) are pivotal in the transition to a clean energy economy, promising green and efficient hydrogen production. Understanding the complexities of overpotentials within PEWEs is crucial for optimizing their performance and efficiency1. For PEWEs to attain the highest possible system efficiency, considering a specific material set and operating parameters, meticulous design is imperative. The goal is to achieve a high reaction rate that is not only optimal but also uniform across the entire active surface area of the electrodes. This uniformity should ideally be maintained with minimal sensitivity to variations in operating conditions.This study focuses on the contribution of different overpotential categories and reveals their spatial variations through in-situ analysis to understand and overcome the issues caused by non-uniformity in reaction. Employing a segmented cell and printed circuit board (PCB) approach2, current density distribution (CDD) and distributed area-specific resistance (DASR) measurements are obtained, enabling a detailed examination of the mass transport overpotentials (as shown in Figure 1). Typically, in the fuel cell community, modeling mass transport limitations is thought of as diffusion-limited transport3. In the anode porous transport layer (PTL), oxygen accumulation (i.e. product transport limitation) can cause membrane dry-out and an increase in ASR4. The experimental results elucidate the intricate relationship between ohmic and mass transport losses, emphasizing the deep coupling induced by the two-phase nature of the system. Figure 1 shows that, as mass transport limitation grows, the highest increase in overpotential comes from mass transport-coupled ohmic losses. Beyond conventional diffusion limited mass transport losses, bubble dynamics significantly impact local hydration and ASR, highlighting the need for a holistic approach in device optimization5.This research contributes to the understanding of PEWE dynamics, providing a basis for informed engineering decisions. By unraveling the intricate interdependencies between overpotentials, this work empowers the efficient utilization of expensive catalyst materials. Moreover, it offers insights into mitigating the challenges associated with bubble removal, accomplishing this with lower pumping requirements that scale non-linearly. These insights enrich both theoretical understanding and are paramount for enhancing system efficiency and economic viability of PEWEs, thereby advancing the prospect of sustainable hydrogen production. Reference N. Danilovic and I. Zenyuk, Electrochem. Soc. Interface, 30 (2021).F. H. Roenning, A. Roy, D. S. Aaron, and M. M. Mench, J. Power Sources, 542, 231749 (2022) https://linkinghub.elsevier.com/retrieve/pii/S037877532200742X.A. Z. Weber et al., J. Electrochem. Soc., 161, F1254–F1299 (2014) https://iopscience.iop.org/article/10.1149/2.0751412jes.C. Immerz, B. Bensmann, P. Trinke, M. Suermann, and R. Hanke-Rauschenbach, J. Electrochem. Soc., 165, F1292–F1299 (2018).P. Satjaritanun et al., iScience, 23, 101783 (2020) https://doi.org/10.1016/j.isci.2020.101783. Figure 1

Investigating the Effects of Anode Catalyst Conductivity and Loading on Performance for Anion Exchange Membrane Water Electrolysis

Water electrolysis (WE) is a promising route for carbon-free production of green hydrogen, which is increasingly considered to be an important energy carrier. Among the low-temperature WE technologies, anion exchange membrane water electrolysis (AEMWE) has the potential advantages of high operating current densities and low cost due to the use of zero-gap assembly and platinum group metal (PGM)-free components, respectively [1]. However, significant challenges remain for this technology, including low efficiency, insufficient durability, and poor performance in pure water operation. Recent efforts have yielded improvements in the individual components, particularly the oxygen evolution reaction (OER) catalysts and anion exchange polymers; however, to further improve the performance of the AEMWE, it is also necessary to understand and improve the integration of these materials into the membrane electrode assembly (MEA). Within the MEA, the anode and cathode catalyst layers (CLs) incorporate catalyst and ion-conducting and/or binding polymers, which are deposited onto either the membrane or the porous transport layer. The material properties of each component, as well as the loadings of the catalyst and ionomer and methods of dispersion and deposition, have significant impact on the catalyst layer morphology and uniformity [2]. These CL properties affect the transport of electrons, hydroxide ions, and evolved gases, ultimately controlling the accessibility and utilization of catalyst active sites, with significant consequences for both the performance and durability of AEMWEs.Most of the effort in catalyst development for AEMWE has focused on the OER, aiming to improve sluggish kinetics and improve stability at the highly oxidizing operating potentials. A number of PGM-free catalysts have shown promising activity in rotating disk electrode (RDE) tests, particularly oxides with varying ratios of Ni, Fe, and Co. The activity improvements with tuning the composition are often attributed to the increase in surface area and shift in redox potentials for the in-situ conversion to higher valence sites with higher intrinsic OER activity. However, these activity trends do not always translate into device-level performance because of differences in the operating environment, e.g. hydroxide concentration, reaction rates, etc. In the AEMWE, where typically an order of magnitude higher loadings and current densities are required, the challenges of conductivity and mass transport to allow for full site access are exacerbated. Electrochemical impedance spectroscopy (EIS) is a valuable tool for understanding the origin of voltage losses in the AEMWE, typically broken down into ohmic, kinetic, and mass transport losses. Recently, a transmission line model has been applied to non-Faradaic EIS to isolate another contribution to voltage loss: the catalyst layer resistance (CLR), which corresponds to in-plane and through-plane electronic and ionic resistances in the catalyst layer [3]. Using this approach, the roles of intrinsic catalyst activity and catalyst layer properties can be decoupled, helping to guide materials integration efforts.In this study, we investigate two types of OER catalyst materials, NiFe- and Co-based oxides. Within each composition, the catalysts have similar active sites but vary in electronic conductivity, crystal structure, and surface area. By comparing the AEMWE performance at different catalyst loadings, we are able to determine the role of catalyst properties in determining active site utilization and electronic resistance within the catalyst layer. Figure 1 shows the trends in performance and ohmic and catalyst layer resistance as a function of loading for high-conductivity (Co@Co3O4) and low-conductivity (Co3O4) catalysts. Despite nominally having the same surface chemistry and similar particle morphology, they display both different performance and different trends with loading. Using in-situ EIS diagnostics and ex-situ characterization of the catalyst layer morphology, the relationship between catalyst utilization, CLR, and the physical properties of the catalyst and catalyst layer is elucidated. This work provides insight into the catalyst properties that most strongly affect AEMWE performance, as well as how materials integration strategies must be tailored based on these properties.